Project Summary Aging is the strongest known risk factor for Alzheimer's disease (AD), the most common cause of dementia in older individuals. However, effective AD therapies remain elusive, which underscores the need to better understand disease etiology and its co-occurrence with advanced age. Apolipoprotein E (apoE) is a major lipid carrier in the brain that transports cholesterol and other lipids such as sulfatide (a key component of myelin) between brain cells. APOE polymorphic alleles are the main genetic determinants of AD risk (e.g., APOE4 allele is the strongest genetic risk factor for AD). APOE alleles are also the major genetic determinants of healthy aging and longevity (e.g., APOE2 is found at a high frequency among centenarians). Previous studies from our group have shown that (1) sulfatide is exclusively transported by apoE particles in an isoform- dependent manner; (2) sulfatide content is specifically and dramatically reduced at the earliest clinically recognizable stages in the brains of AD human subjects and animals; (3) sulfatide depletion in AD is accelerated by amyloid beta; (4) brain sulfatide is regulated in an age dependent manner decreasing during old age in both humans and rodents; (5) sulfatide-depleted mice display myelin and axonal abnormalities, extensive astrogliosis, and reduced lifespans; and (6) sulfatide mediates neuronal uptake of apoE. These findings led us to hypothesize that apoE2 neuroprotective effects in the aging brain are driven by the capacity of apoE2 particles to carry less sulfatide content, compared to other isoforms, which consequently leads to higher brain sulfatide levels and lower abilities to bind apoE receptors. To test this central hypothesis, we propose three Specific Aims: 1) To investigate the association of brain apoE and lipid content from human and mouse APOE2 and E3 carriers with neuroplasticity, markers of brain aging (white matter integrity and brain atrophy), cognitive performance, and/or AD onset at different age groups; 2) To determine if sulfatide (or apoE) content affects neuroplasticity, white matter integrity, and cognitive performance in the aging mouse brain; and 3) To elucidate the molecular mechanism(s) by which brain sulfatide metabolism underlies the neuroprotective effects of apoE2 in AD and aging. To the best of our knowledge, the proposed research (in response to RFA-AG-18-022: ?Understanding the Effects of ApoE2 on the Interaction between Aging and Alzheimers Disease?) is the first to study the potential role of sulfatide in apoE2-mediated neuroprotection. If our hypothesis is borne out, this work may unravel the mechanism(s) by which apoE2 promotes healthy aging in the brain and lead to new therapeutic targets (i.e., sulfatide and the proteins involved in its metabolism) to treat AD and aging-related cognitive decline.